Welcome to iMore! If the iPad is like the iPhone, there's no way to rearrange the photos in an album created in the stock photo app. You can start over. Here's what I would do. Create a new album and place your photos in the arrangement you want them in. You can easily find them from in the original album. After completion, then delete the old album. If you really want more control over your photos and don't want to use cloud based apps like Box, Dropbox and OneDrive, try an app called MyPhoto. It's great for organizing photos in albums. It has many features I like. Once you put photos in MyPhoto, then you can delete them from your camera roll.